1. Field of the Invention
This invention relates to a method for the impartation of a durable primary color of blue, green, red, brown, white, etc. to the surface of an aluminum or aluminum alloy material (hereinafter referred to as "aluminum material").
2. Description of the Prior Art
As methods for the coloration of an aluminum material already treated for the formation of an anodic oxide coating thereon, various methods have been known to the art such as the so-called electrolytic coloring method which comprises immersing an anodized aluminum material in an electrolytic solution containing at least one metal salt selected from inorganic acid salts such as nitrates, sulfates, phosphates, hydrochlorides, chromates and organic acid salts such as oxalates, acetates, and tartrates of such metals as Ni, Co, Cr, Cu, Cd, Ti, Mn, Mo, Ca, Mg, V, Pb, and Zn, and subjecting the immersed aluminum material to the electrolysis with alternating current thereby causing a relevant metal, metal hydroxide, or metal oxide to be deposited within micropores in the anodic oxide coating and consequently imparting the corresponding color to the surface of the aluminum material [as disclosed in Japanese Patent Publication SHO 38(1963)-1,715 and SHO 54(1979)-23,661, for example]and the method which comprises combining a direct current, an alternating current, a waveform similar thereto, or a deformed waveform in the process of secondary electrolysis. Further, the so-called integral color anodizing method which comprises anodizing an aluminum material to form a colored anodic oxide coating on the aluminum material by virtue of the particular kind of an electrolytic solution or the action of the element to be incorporated in the aluminum material has been known to persons skilled in the art [as disclosed in U.S. Pat. No. 3,031,387, U.S. Pat. No. 3,143,485, etc.]. Besides the methods which resort to such electrochemical treatments as described above, the methods of immersion coloration using baths of inorganic compounds or organic dyes have been well known in the art.
By the electrochemical treatment such as for the electrolytic coloring or integral color anodizing mentioned above, it is not easy to impart a clear primary color to aluminum materials such as facing materials in buildings which have been furnished for the fulfilment of the quality of durability with an anodic oxide coating of a medium thickness (not less than 9 .mu.m, for example). It is said that the anodic oxide coating entails a decline in the quality of practical durability when it is given a treatment for increasing their micropores in size and number by combining a direct current, an alternating current, a waveform similar thereto, a deformed waveform, or voltage in the process of secondary anodic oxidation intended to ensure impartation of a primary color.
In contrast, by the method of immersion coloration, it is not easy to impart a durable and clear color to the aluminum material because a coloring substance is predominantly deposited in the entrance parts of the micropores of the anodic oxide coating and, during the aftertreatment as by washing with water, the deposited coloring substance is removed from the micropores to the extent of impairing the resistance to corrosion and resistance to light of the colored aluminum material.